The present invention relates to a method and apparatus for creating images in photosensitive material, and more specifically to a method and apparatus for creating photomasks using a raster scan based exposure system in which the image formed in the photosensitive resist material and, in turn, the attenuator material, has improved definition of inside corners.
Photomasks are used in the semiconductor industry to transfer microscale images defining a semiconductor circuit onto a silicon or gallium arsenide substrate or wafer. The process for transferring the image from a photomask to a silicon substrate or wafer is commonly referred to as lithography or microlithography. Generally, a photomask is comprised of a substrate and an attenuator. A typical or binary photomask is comprised of a quartz substrate and a chrome attenuator. The pattern of the attenuator material is representative of the image desired to be formed on a silicon wafer. To develop an image on a silicon wafer, a layer of photosensitive material (i.e., photoresist) is applied to a silicon substrate. The photomask is placed between the silicon wafer and a light or other energy source. The light or energy is inhibited from passing through the areas of the photomask in which the attenuator is present. The solubility of the photoresist material is changed in areas exposed to the light or energy. In the case of a positive photolithographic process, the exposed photoresist becomes soluble and can be removed. In the case of a negative photolithographic process, the exposed photoresist becomes insoluble and unexposed soluble photoresist is removed.
After the soluble photoresist is removed, the latent image is transferred to the substrate by a process well known in the art which is commonly referred to as etching. Once the pattern is etched onto the substrate material, the remaining resist is removed resulting in a finished product.
The pattern formed in the attenuator material defining the image to be transferred to the silicon substrate is produced by a similar process. The desired image to be created on the photomask is initially defined by an electronic data file typically generated by a computer aided design (CAD) system. The data file is loaded into an exposure system which scans an electron beam (E-beam) in a raster fashion across an unexposed or blank photomask which, as shown in FIG. 1A, is comprised of a layer of photosensitive material 2, a layer of attenuator material 4, and a substrate 6.
Examples of raster scan exposure systems are described in U.S. Pat. No. 3,900,737 to Collier and U.S. Pat. No. 3,801,792 to Lin. Each finite location in which the E-beam can be positioned is referred to as an addressable location or pixel. Typically, the physical dimensions of an addressable location, and hence the resolution of the exposure system, are defined by diameter or width of the E-beam. As the E-beam is scanned across the blank photomask, the exposure system energizes the E-beam at addressable locations defined by the electronic data file. As shown in FIG. 1B, the unexposed, soluble photoresist is removed and the exposed, insoluble photoresist material 8 remains adhered to the attenuator material 4. As shown in FIG. 1C, the attenuator material which is no longer covered by the photoresist material is removed by a well known etching process leaving only portions of attenuator material 10 which correspond to the hardened photoresist material 8. As shown in FIG. 1D, the hardened photoresist material is subsequently removed leaving the attenuator material 10 conforming to the image defined in the data file remaining on the substrate. The above process is described utilizing a positive photoresist material, however, the same process is applicable if negative photoresist (i.e., the exposed resist becomes soluable) is utilized.
It will be appreciated that the more accurately the attenuator pattern reflects the desired image defined in the electronic data file, the more accurately the image produced on the silicon substrate will reflect the desired image. However, the pattern formed in the attenuator material by the raster scan exposure system is not a perfect reproduction of the desired image defined by the electronic data file. Factors such as the circular beam diameter, whether or not an edge is scanned or unscanned, and dose proximity effects all effect the quality of the image formed in the photosensitive and attenuator materials. As will be described herein, the degradation in correlation between the desired and created images is most pronounced at xe2x80x9cinside cornersxe2x80x9d locations.
The shortcomings of the prior art process are depicted in FIG. 2. In FIG. 2A, images 12 and 14 represent the desired image to be produced on a photomask as defined by the electronic data file. FIG. 2B depicts the raster scanning process of the E-beam with the vertical arrows depicting the direction of the E-beam scan. When the E-beam 16 is positioned at addressable locations defining the desired images 12 and 14, the E-beam 16 is energized thereby exposing the corresponding portions of photosensitive resist material. In practice, the E-beam is not de-energized when passing between addressable locations which are both intended to be exposed. FIG. 2C depicts the resultant image created in the photoresist material and hence the attenuator material. As shown in FIG. 2C, the inside corners 18 of the image are rounded and do not accurately reflect the image defined by the electronic data file, while outside corners 20 more closely represent the image defined in the data file.
FIG. 3 demonstrates the difference in reproducibility of the desired image at inside corner and outside corner locations. FIG. 3A is an enlarged depiction of a typical outside corner 20. When E-beam 16 reaches the horizontal boundary 22 of the desired image, the beam is de-energized and repositioned for the next scan line. Horizontal boundary 22 is considered an unscanned edge because the E-beam 16 is not passed along the boundary in an uninterrupted scan. Conversely, vertical boundary 24 is considered a scanned edge because E-beam 16 is passed along the boundary in an uninterrupted scan. As the beam is scanned in the scan line directly adjacent to vertical boundary 16, it passes beyond the horizontal boundary 22 thereby forming a clean or sharp corner in the photosensitive resist material.
FIG. 3B is an enlarged depiction of a typical inside corner 18. When E-beam 16 reaches the horizontal boundary 22 of the desired image, the beam is de-energized and, as such, horizontal boundary 22 is considered an unscanned edge. Although vertical boundary 24 is considered a scanned edge (i.e., the beam is passed along the boundary in an uninterrupted scan) the beam does not extend past the horizontal boundary 22. It will therefore be appreciated that an inside corner 18 will be less sharp or more rounded than an outside corner 20. The deviation from the desired image is propagated from the photomask to the silicon substrate thereby degrading the performance or capabilities of the semiconductor circuitry.
Prior art references have considered the limitations of a raster scan exposure system for use in creating photomasks. For example, U.S. Pat. No. 4,498,010 issued to Biecheler addresses the problem of producing images in photosensitive material in which the edge of an the image is between two rows of addressable locations. To overcome system resolution incompatibility, every other addressable locations of the scan line that is beyond the desired image is exposed to the particle beam. After exposure, the areas or valleys between the alternately exposed addressable locations are allegedly filled and the feature width is approximately one-half a addressable location width.
In the related field of optical proximity correction technology, U.S. Pat. No. 5,663,893 to Wampler describes the use of serifs to more accurately produce a desired image on the silicon substrate. Serifs are selective distortions of the features of the attenuator pattern which improve the quality of resulting images through the use of well known micro-optic principles (e.g., diffraction). The use of serifs are generally described with reference to FIG. 4. As shown in FIG. 4A, the desired image 26 to be created on a silicon substrate is a square. With reference to FIG. 4B, the desired image to be formed in the photosensitive resist material includes serifs 28 at each corner, each serif including inside corners 30a-30c. FIG. 4C represents the image created in the photoresist material and attenuator material using the raster scan E-beam process described above. As shown, inside corners 34a-34c of the serifs 32 are rounded thereby reducing their micro-optic effects and thus the quality of the image formed on the silicon substrate. As will be discussed further herein, the present invention can be used in conjunction with serifs to improve the quality of the created image.
Accordingly, it is the object of the present invention to provide a method for creating an image in a photosensitive material which more accurately reflects the desired image as defined by an electronic data file.
It is a further object of the present invention to provide a method for creating photomasks using raster scan exposure systems wherein the images formed on photomasks produced by the method more accurately reflect the desired image as defined in an electronic data file.
It is a further object of the present invention to provide a method for creating semiconductor wafers using raster scan exposure systems wherein the images formed on the wafers produced by the method more accurately reflect the desired image as defined in an electronic data file.
It is still a further object of the present invention to provide a raster scan exposure system which produces images in photosensitive materials which more accurately reflects the desired image to be produced as defined by an electronic data file.